1. Field of the Invention
This application relates generally to optical coherence tomography (OCT) imaging methods and apparatuses and, more specifically, to a method capable of detecting missampling in a swept source OCT imaging system.
2. Description of Related Art
OCT is an imaging technique capable of non-invasively acquiring sub-surface images of a subject at micrometer resolutions. Given such high resolutions, OCT is a preferred imaging technique in medical fields. OCT works by detecting the interference light of a light signal passing through an object and comparing it to a reference light signal. A major advance in this technique is frequency domain OCT (FD-OCT) because it is several hundred times faster than conventional time domain OCT (TD-OCT). FD-OCT includes both spectral domain OCT (SD-OCT) in which the interference light is detected through spectral decomposition, and swept source OCT (SS-OCT) in which interference lights of various wavelengths, interferograms, are obtained using a wavelength-swept light source.
However, because the spectral interferograms are detected in real values, reconstruction of the image consists of a true image and a complex conjugate artifact displayed as a mirror image of the true image. As a result, the measurement depth of the object must be shifted and the possible imaging depth of FD-OCT is shrunk by half. Currently, various phase shifting, modulation, and alternation techniques exist to remove or suppress the problem of complex conjugate artifacts. These techniques are described, for example, in U.S. Pat. No. 8,564,788, which is incorporated by reference herein.
One drawback with such techniques is that they require phase stability in the detected interferogram. In SS-OCT, sequential spectral interferograms are acquired along the lateral direction to construct the axial lines (A-lines) that form a 2D OCT image (B-scan). These interferograms are represented graphically by plotting the intensity of the interference signal in k-space, where k is the wavenumber (the inverse of wavelength) of a frequency in the swept source bandwidth. Due to nonlinear sweeping of wavenumbers by the light source in SS-OCT, an external linear k-clock synchronized to the light source is commonly used to ensure the linearity of the sampling points for the interferogram in k-space. In this way, the data points of the interferogram are acquired at a set of wavenumber points with fixed sampling intervals. In practice, when forming a 2D FD-OCT image, each A-line is first constructed by performing a Fourier transform of the interferogram. It is the Fourier transform that requires the sampling to be linear in k-space, so that the interferogram in k-space is transferred to A-line in depth without errors caused by k-space non-linearity.
Despite the addition of a k-clock, there is still the problem that an asynchrony exists between the k-clock and a sweep trigger used to trigger the light source to begin sweeping frequencies for each A-line scan. This asynchrony results in the uncertainty of the starting wavenumber of the interferogram, which in turn affects phase sensitive measurements by introducing depth dependent phase jittering among A-lines.
To address the asynchrony problem, for example, “Doppler velocity detection limitations in spectrometer-based versus swept-source optical coherence tomography” (Aug. 1, 2011) by Hendargo et al. introduced a fiber Bragg grating (FBG) optical notch filter to the OCT system to ensure a stable starting wavenumber; “Phase-sensitive swept-source optical coherence tomography imaging of the human retina with a vertical cavity surface-emitting laser light source” (Feb. 1, 2013) by Choi et al. similarly introduced an FBG to be used as a wavenumber reference signal so that fluctuations can be numerically compensated for during postprocessing by shifting the interferograms based on the reference signal; and “Three-dimensional anterior segment imaging in patients with type 1 Boston Keratoprothesis with switchable full depth range swept source optical coherence tomography” (August 2013) by Poddar et al. illustrated different locations to place an FBG in the OCT system to generate a reference signal.